Method for making a stent

ABSTRACT

A stent comprising a coil including a plurality of arcuate sections that alternate directions around a central axis, each arcuate section including a pair of curved turns joined by a cusp, and the cusps of adjacent arcuate sections intermeshing and defining at least one region of overlap, which in turn describes a helix around and along the length of the coil. In the preferred embodiment, there are two regions of overlap, which together describe a double helix. In another preferred embodiment, the stent is bifurcated so as to support a branched vessel or the like. 
     A method for forming a stent, including the steps of providing a flat sheet of material, chemically etching said sheet to form a blank, and forming said blank into a cylindrical coil. The coiling step is preferably carried out on a plurality of rollers.

This is a continuation of co-pending application(s) Ser. No. 08/367,239filed on Dec. 16, 1994, now U.S. Pat. No. 5,607,445 which is thenational phase of International Application PCT/US93/05823 filed on Jun.16, 1993 which designated the U.S. and which is a continuation-in-partof Ser. No. 07/900,896 filed on Jun. 18, 1992 now U.S. Pat. No.5,342,387.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to supports for collapsed or occludedblood vessels, and more particularly, to a coiled wire stent forinsertion and expansion in a collapsed or occluded blood vessel. Stillmore particularly, the present invention relates to a coiled, bifurcatedstent which supports a Y-shaped juncture of two blood vessels.

The present invention further relates to methods and apparatus formanufacturing artificial supports for blood vessels and moreparticularly, to methods for making a wire coil having certain desiredproperties. Still more particularly, the present invention disclosesmethods and apparatus for preparing a continuous loop and forming theloop into a cylindrical shape having a desired configuration.

BACKGROUND OF THE INVENTION

A typical wire stent for insertion and expansion in a collapsed oroccluded blood vessel is shown in U.S. Pat. No. 4,800,882 and includes acoiled wire having a plurality of curved sections that are formed into agenerally circular configuration. Adjacent curved sections are joined bya bend so that a series of alternating opposing loops are formed. Thestent has a cylindrical shape with a longitudinal opening through whicha folded balloon catheter is inserted. The opposing loops are tightlycontracted about the catheter so that the cylindrical shape has anoverlapping region in which portions of adjacent loops circumferentiallyoverlap. The loops are arranged so that when the balloon catheter isinflated, adjacent loops diverge circumferentially relative to eachother, thereby decreasing the width of the overlapping region whileincreasing the diameter of the cylindrical shape. As the diameter of thecylindrical stent increases, the stent engages the inner surface of theblood vessel.

In operation, the stent is deployed at its desired position within thevessel in its collapsed state, by threading the balloon catheter tip thevessel from an incision some distance away, and then expanded to itsexpanded state, for supportive engagement with the interior of thevessel wall.

The prior art stents have several deficiencies. As shown in FIG. 7 ofU.S. Pat. No. 4,800,882, the alternating bends are aligned in relationto the longitudinal axis of the stent such that upon expansion of thestent as shown in FIG. 8, the opposing loops may be expanded such that alongitudinal gap appears between the opposing bends of the loops,leaving a longitudinal unsupported area along the occluded blood vessel.Such an unsupported area is undesirable. Further, when it is desired tosupport a branched section of a blood vessel without obstructing thepassageway of the vessel, it is necessary to utilize severalconventional stents to support the main vessel and the adjacent twobranch vessels. Deployment of multiple stents requires an extendedmedical procedure, and may produce unsatisfactory results if any of thestents migrates away from the juncture, leaving one leg of the Y-shapedjuncture of the vessels unsupported. Additionally, the stents of theprior art often require the application of heat, torsional force, or ashortening in length in order to attain their expanded state.

Alternatively, stents having no longitudinal gap may be comprise spiralcoils, or other configurations that are radially expandable and providethe desired circumferential support for a collapsed vessel.

Because of the asymmetrical nature of many of the desired coilconfigurations, standard manufacturing methods are inapplicable. Thus,stents such as that disclosed in U.S. Pat. No. 4,800,882, involve a highdegree of labor to produce. The present invention discloses means andapparatus for producing a desired stent quickly and easily.

The present invention overcomes the deficiencies of the prior art.

SUMMARY OF THE INVENTION

The stent of the present invention comprises a coil including aplurality of arcuate sections that alternate clockwise andcounterclockwise directions around a central longitudinal axis. Eacharcuate section includes a pair of curved turns joined by a cusp. Thecusps of adjacent arcuate sections intermesh, thereby defining at leastone region of overlap, which in turn describes a helix around and alongthe length of the coil. In the preferred embodiment, there are tworegions of overlap, which together form a double helix.

The present invention further discloses a Y-shaped, bifurcated stent.The bifurcated stent comprises three coils, each constructed accordingto a preferred coil pattern, joined so as to form an unobstructedsupport for a branched vessel.

The stent of the present invention is radially expandable without theuse of heat, torsional forces, or shortening of the stent, and isconstructed to provide a region of enhanced support which wrapshelically around the stent. The branched stent fills the need for areliable device which is simple to install and effectively supports abranched blood vessel.

The present invention further discloses a double-spiral stent that maycomprise either a single coil or a bifurcated coil, and a rib-cage typestent that includes a longitudinal spine supporting a plurality oflooped ribs thereon.

Also disclosed is a method for making the foregoing stents that israpid, repeatable and economical. The method of the present invention isnot labor intensive and is capable of producing even stents that do nothave a linear axis of symmetry. The present method includes creating acontinuous loop, or blank, by photoetching a sheet of material. Theblank produced by the photoetching technique has no ends or joints andis therefore superior for internal applications because the likelihoodof a puncture or other damage is minimized.

The present method further includes rolling the continuous blank betweena series of rollers, to form a cylindrical coil. According to thepresent invention, the precise configuration of the coil is determinedby the shape of the blank that is rolled. Thus, single-helix,double-helix and spiral configured stents may be constructed by rollingaccording to the present invention.

Alternatively, a method for constructing the desired coil shape by handis disclosed. The manual method comprises forming each loop of the coilaround a mandril by individually pulling the cusps of the coil intoplace. This manual method may of course be automated to increase speedand efficiency of production.

Other objects and advantages of the present invention will appear fromthe following description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of a preferred embodiment of the invention,reference will now be made to the accompanying drawings wherein:

FIG. 1 is a perspective view of a double-helix nonbifurcated stentaccording to the present invention.

FIG. 2 is an end view of the double-helix nonbifurcated stent of FIG. 1.

FIG. 3 is an enlarged view of two full loops of the double-helix stentof FIG. 1.

FIG. 4 is a close-up view of two full loops of the double-helix stent ofFIG. 1.

FIG. 5 is a perspective view of a single-helix nonbifircated stent.

FIG. 6 is a perspective view of a double-helix bifurcated stentaccording to the present invention.

FIG. 7 is a perspective view of a bifurcated stent in which the majorcoil is a double-helix and the two minor coils are single-helix.

FIG. 8 is a side elevational view of the stent of FIG. 6 in a collapsedstate, mounted on a balloon catheter within a blood vessel.

FIG. 9 is a side elevational view of the stent of FIG. 6 deployed withina bifurcated vessel and partially expanded.

FIG. 10 is a perspective view of a first nonbifurcated cross-over stent.

FIG. 11 is a perspective view of a second nonbifurcated cross-overstent.

FIG. 12 is a perspective view of a bifurcated zig-zag stent according tothe present invention.

FIG. 13 is an enlarged view of two of the loops of the zig-zag stent ofFIG. 12.

FIG. 14 is an isometric view of a nonbifurcated ribbon stent.

FIG. 15 is an isometric view of a bifurcated ribbon stent.

FIG. 16 is a perspective view of a double-spiral stent made according tothe present invention;

FIG. 17 is a perspective view of a bifurcated double-spiral stent madeaccording to the present invention;

FIG. 18 is a perspective view of a bifurcated stent in which the majorcoil is a double-spiral and the minor coils are single spirals;

FIG. 19 is an perspective view of a continuous loop blank that may beused to form the stents of FIGS. 16 and 17;

FIG. 19A is a cross section of the blank of FIG. 19 taken along linesA--A of FIG. 19;

FIG. 20 is an elevational view of a continuous loop blank that may, beused to form the stents of FIGS. 1 and 6;

FIG. 21 is a perspective view of a backbone blank;

FIG. 22 is an end view of an apparatus that can be used according to thepresent invention to produce the stents of FIGS. 1, 5, 6, 7, 16, 17, and21;

FIG. 23 is a perspective view of the apparatus of FIG. 22 forming theloop of FIG. 19 into the stent of FIG. 16;

FIG. 24 is a perspective view of the apparatus of FIG. 22 forming theloop of FIG. 20 into the stent of FIG. 1; and

FIG. 25 is a perspective view of a stent formed from the rib-cage blankof FIG. 21.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Double Helix Stent

Referring initially to FIGS. 1 and 2, there is shown a preferredembodiment of a stent 10 according to the present invention. Stent 10 ismade of a single length of wire having a mid-point at 12 forming twowire legs 14, 16 of approximately equal length. Legs 14, 16 are bentinto a double-helix coil 22 as shown, forming individual spiral wireshell halves 18, 20 respectively. One end 24 of coil 22 includesmid-point 12, and the other end 26 of coil 22 includes the terminal ends28, 30 of wire legs 14, 16. Terminal ends 28, 30 are connected atjuncture 32 on coil end 26, such as by soldering or the like. Upon thejoining of terminal ends 28, 30, coil 22 effectively consists of asingle continuous wire 34. The two wire shell halves 18, 20 are curved,as shown in the end view of FIG. 2, so that stent 10 is generallycylindrical in shape with a generally circular opening 36 formedtherein. Stent 10 is shown having a central longitudinal axis 38.

Referring now to FIGS. 1, 3, and 4, each individual spiral wire shellhalf 18, 20 includes a series of alternating clockwise andcounterclockwise arcuate sections. For purposes of description, thearcuate sections have been severed in FIG. 3 to better illustrate suchsections. The clockwise direction relative to the axis 38 has beenarbitrarily selected and is indicated by the arrow CW. Wire shell half18 includes alternating clockwise and counterclockwise arcuate sections40, 42, respectively, and opposed wire shell half 20 includesalternating counterclockwise and clockwise arcuate sections 50, 52,respectively. Clockwise arcuate section 40 is typical of the otherarcuate sections and includes two adjacent curved turns 44, 46 of wirejoined by a bend or cusp 48. Likewise, counterclockwise arcuate section50 of shell half 20 includes two adjacent curved turns 54, 56 joined bya cusp 58.

As best seen in FIG. 1, wire leg 14 forms shell half 18, comprisingclockwise arcuate sections 40a, 40b, 40c, etc., with cusps 48 pointingin the clockwise direction and counterclockwise arcuate sections 42a,42b, 42c, etc., with cusps 48 pointing in the opposite counterclockwisedirection. Likewise, wire leg 16 forms shell half 20 comprisingcounterclockwise arcuate sections 50a, 50b, 50c, etc. with cusps 58 andclockwise arcuate sections 52a, 52b, 52c, etc. with cusps 58. Theclockwise arcuate sections 40 of shell half 18 are in phase with thecounterclockwise arcuate sections 50 of shell half 20 so that theclockwise arcuate sections 40 of half 18 intermesh and extend betweencounterclockwise arcuate sections 50 of half 20. The same is true forcounterclockwise arcuate sections 42 of half 18 and clockwise arcuatesections 52 of half 20.

Referring now to FIG. 4, the intermeshing of arcuate sections 40, 50 and42, 52 creates two regions of overlap in coil 22. Clockwise arcuatesections 40 and counterclockwise arcuate sections 50 create a firstoverlap region 60 and counterclockwise arcuate sections 42 and clockwisearcuate sections 52 create a second overlap region 70. Regions ofoverlap 60, 70 have diametrically opposed centerlines 62, 72,respectively.

Referring now to FIGS. 1 and 4, the extent of the regions of overlap 60,70 will vary with the size of the blood vessel in which the stent 10 isdeployed. The extent of the region of overlap 60, 70 is maximized in thecontracted position of the stent 10 and is minimized in the expandedposition of the stent 10. The intermeshing of adjacent arcuate sections40, 50 and 42, 52 defines an angle a at axis 38, shown in FIG. 1.Preferably, in the contracted position, α is at least five longitudinaldegrees. In the Figures, an intermeshing of only a few longitudinaldegrees is shown, but it will be understood that the degree ofintermeshing can be increased without departing from the spirit of theinvention and is actually increased when the stent is used. Cusps 48, 58of arcuate sections 40, 42, 50, 52 shift circumferentially with eachturn. In this manner, regions of overlap 60, 70 describe a double-helixaround coil 22, best demonstrated by reference centerlines 62, 72, shownin FIG. 1. The advantages of this construction will become apparent fromthe discussion below.

Preferably, stent 10 is constructed of wire, although any suitablematerial may be substituted. The wire comprising stent 10 is malleable,preferably from the group consisting of annealed stainless steel,tungsten and platinum. This malleable material must be sufficientlydeformable to allow shell halves 18, 20 to expand radially when radiallyoutward pressure is applied by the inflation of the membrane thatcomprises the standard balloon catheter. Because the stent materialdeforms plastically, rather than elastically, the stent 10 retains theenlarged diameter after the balloon is deflated.

The material has sufficient strength and stiffness, however, to avoidthe stent 10 being displaced during insertion and to avoid the adjacentarcuate sections 40, 50 and 42, 52 being forced into an overlyingrelation. Further, the stent 10 has sufficient strength and stiffness toallow it to maintain its position in the vessel passageway and to resistbeing dislodged after the catheter has been deployed. One example of asuitable wire has an outer diameter of 0.018 inches and is stainlesssteel AISI 315 alloy. Alternately, the stent 10 of the present inventioncan be constructed of a memory metal, such as Nitinol, that resumes aparticular original shape, following deformation, when heat is applied.

In a preferred embodiment, the surface of the stent is coated with abiocompatible substance, preferably a biolized collagen/gelatin compoundsuch as those discussed in Characterization of Rehydrated Gelatin Gels,Emoto. et al., Artificial Organs, 15(1):29-34, 1991 and incorporatedherein by reference. The coating serves to increase biocompatibility ofthe stent and aid in blood flow around the device. The coating is a 5%glutaraldehyde cross-linked dried gelatin coating which can be appliedto a texturized surface, dehydrated, sterilized, and stored dry. Thistype of gel, when applied as a film, provides a smooth, biochemicallystable protein coating with non-pseudointima properties, very littleplatelet adhesion, and high blood compatibility.

To deploy a stent such as the stent 10 of FIG. 1 in a blood vessel, thestent is radially contracted or compressed until it assumes a outerdiameter which is calibrated to allow insertion into a particular vesselpassageway. Typically, this means an outer diameter on the order of 3millimeters. With regard to stent 10, as the stent is compressed,regions of overlap 60, 70 widen and the cusps 48, 58 are forced intodeeper intermeshing relationship. The stent 10 in its contracted stateis threaded onto a balloon catheter (not shown) prior to deployment inthe vessel. The compressed stent 10 and catheter are inserted at anincision in the vessel and threaded up the vessel on a wire guide to theplace of deployment. At that point, pressure is applied to the balloonto expand it within the stent. As the balloon is inflated, the clockwiseand counter-clockwise arcuate sections 40, 50 and 42, 52 expandradially, reducing the width of overlap regions 60, 70 until the desiredcircumference is attained. Thus, the effective diameter of stent 10 isincreased without thermal expansion, application of torsional forces tothe stent, or a reduction in overall length of the stent.

Single Helix Stent

Referring now to FIG. 5, there is shown an alternate nonbifurcated stent61 comprising a single helix coil 64. According to this embodiment, awire is bent into a series of alternating clockwise and counterclockwisearcuate sections 63, 65, formed by turns 66 and cusps 68, such that oneregion of overlap 71 is formed. The ends 67, 69 of the wire are locatedat opposite ends of coil 64. As described above with regard to stent 10,arcuate sections 63, 65 are constructed so that region of overlap 71shifts longitudinally with each successive turn 66 and forms a spiralaround coil 64. Because there is only one region of overlap, 71, coil 64is referred to as a single-helix coil. Stent 61 can be deployed in themanner discussed above with regard to stent 10.

Referring now to FIG. 6, there is shown a preferred bifurcated stent 80according to the present invention. Bifurcated stent 80 includes a majorcoil 82 and two minor coils 84, 86. In practice, major coil 82 may beplaced, for example, in the aortic vessel and minor coils 84, 86 in theiliac vessels. As with stent 10, bifurcated stent 80 comprises a singlecontinuous wire 34, and each of coils 82, 84, 86 comprises a part of thewire 34. Major coil 82 has the same double helix pattern as coil 22 ofstent 10, with the exception that wire legs 14, 16 are extended to formminor coils 84, 86, respectively, which are also coiled in the doublehelix pattern of coil 22. As in coil 22, the terminal ends 88, 90 ofwire legs 14, 16 are joined at a juncture 92. Hence, a single wire loopis able to define and flexibly support a branched vessel withoutobstructing flow therethrough.

Referring now to FIG. 7, an alternate bifurcated stent 81 comprises onemajor coil 83 constructed in the manner of double-helix coil 22 of stent10 shown in FIG. 1, and two minor coils 85, 87 constructed in the mannerof single-helix coil 64 of stent 61 shown in FIG. 5. As with bifurcatedstent 80, bifurcated stent 81 can be constructed from a single piece ofwire. In stent 81, wire legs 14, 16 terminate at ends 88, 90, which maybe joined to coils 85, 87, as shown at 91, individually formed intoloops (not shown), or otherwise prevented from puncturing the vesselwall.

Deployment of a bifurcated stent is shown in FIGS. 8 and 9, using thestent 80 shown in FIG. 6. Stent 80, in a contracted state on ballooncatheter 112, is threaded up one of the iliac vessels 108, 110 from anincision in the leg, as shown in FIG. 8. When it reaches the juncture106 of vessels 108, 110, the stent 80 is pushed up into the aorticvessel 100 by a guide wire 104, until one of the minor coils 84, 86 ofthe stent 80 is clear of the juncture 106. Then the stent 80, still in acompressed state, is backed down the iliac vessel 108 until it is in itsproper position for expansion. FIG. 9 shows stent 80 in position fordeployment within the vessel juncture 106 and partially expanded.Bifurcated stents of the present invention having a variety of coilpatterns may be deployed in the manner described above with respect tostent 80.

As shown in FIGS. 8 and 9, a tri-wing balloon 112 may be used to inflatestent 80, so that uniform pressure is applied to each coil of stent 80and the coils expand simultaneously. The balloon material is flexible,so that, once deflated, it may be easily removed through any opening inthe stent 80. Preferably, it is removed through an opening where theaortic section branches to form the iliac sections, or through the endof one of the iliac sections.

Cross-Over Stents

Referring now to FIG. 10, an additional single-helix, nonbifurcated coil120, is shown. In coil 120, both wire ends 88, 90 are at one end of thecoil 120, and one region of overlap 130 is formed. In alternateembodiments of coil 120 (not shown), wire ends 88, 90 may be joined asat 32 in FIG. 1, formed into loops, or extended to form integraladjacent minor coils. Region of overlap 130 describes a helix aroundcoil 120, as discussed above with respect to coil 22. Unlike coil 22,however, where the material forms two opposing individual shell halvesthat do not cross, the material of coil 120 forms two generallycylindrical legs 124, 126, each comprising of a series of alternatingclockwise and counter-clockwise sections 132, 134 having clockwise andcounter-clockwise cusps 133, 135, respectively. Each leg 124, 126 hasits own region of overlap, and legs 124, 126 are intermeshed so that theregions of overlap coincide. When intermeshed, legs 124, 126 cross eachother at a series of cross-overs 128. For this reason, coils such ascoil 120 are hereinafter referred to as cross-over coils. It should benoted that in coil 120 clockwise cusps 133 and counterclockwise cusps135 alternate along region of overlap 130.

Coil 120 provides a radially expandable coil with an asymmetrical regionof overlap. The fact that both wire ends 88, 90 are at one end of coil120 makes coil 120 suitable for the construction of either a closedloop, nonbifurcated stent or a bifurcated stent in which the minor coilsare formed from, and are therefore integral with, ends 88 and 90.

Referring now to FIG. 11, an alternate single-helix, cross-over,nonbifurcated coil 120, is shown. In coil 122, as in coil 120, both wireends 88, 90 are at one end of the coil 122, and one region of overlap130 is formed. Region of overlap 130 describes a helix around coil 122,as discussed above with respect to coil 120. As in coil 120, thematerial of coil 122 forms two generally cylindrical legs 124, 126, eachcomprising of a series of alternating clockwise and counter-clockwisesections 132, 134 having clockwise and counter-clockwise cusps 133, 135,respectively. Each leg 124, 126 has its own region of overlap, and legs124, 126-are intermeshed so that the regions of overlap coincide. Whenintermeshed, legs 124, 126 cross each other at a series of cross-overs128. Wire ends 88, 90 of coil 122 may be treated in the same manner asdiscussed above with regard to coil 120.

In coil 122, however, unlike coil 120, a pair of clockwise cusps 133 isfollowed by a pair of counterclockwise cusps 135 etc., defining regionof overlap 130 in a manner different from coil 120. The differencearises in the pitch of the turns of each leg. In coil 120, the turnshave an uneven pitch, in that one leg 126 forms a first pair of adjacentcusps 135a, 133a and then passes behind the other leg 124, which formsthe next pair of adjacent cusps 135b, 133b, and so on. In coil 122, theturns of each leg have a regular pitch, but do not form the alternatingclockwise. counterclockwise pattern of cusps at region of overlap 130.

Zig-Zag Stent

In another alternate embodiment, shown in FIG. 12, a bifurcated stent140 comprises three radially expandable cylinders 142, 144, 146 formedof wire in a zig-zag pattern. The diameter of minor cylinders 144, 146is approximately half the diameter of major cylinder 142, and minorcylinders 144, 146 are mounted adjacent to one another at one end ofmajor cylinder 142, so as to provide support for a branched vesselwithout obstructing fluid flow therethrough. The attachment of minorcylinders 144, 146 to major cylinder 142 may be by any suitable means,such as by forming a loop or hinge to provide a flexible joint, or bysoldering the coils together.

As best seen in FIG. 13, zig-zag stent 140 comprises a plurality ofstraight sections 147 joined by a series of loops 148, with a crossover149 corresponding to each loop 148. The wire reverses directions at eachloop 148, so that each straight section 116 crosses the two adjacentstraight sections at cross-overs 149, forming a zig-zag pattern. A stentof this configuration has no longitudinal gap when expanded.

Stent 140 is radially expandable without thermal expansion or theapplication of torsional forces to the stent. The loops avoid sharpbends in the wire which might otherwise occur between adjacent straightsections, and, by enabling the straight wire sections to be crossed,increase the strength and stability of the stent. Additionally, zig-zagstent 140 can be constructed of a single, continuous piece of wire, withthe wire passing form major cylinder 142 to each minor cylinder 144, 146and back at least once.

Spiral Stent

Referring now to FIGS. 14 and 15, an alternate embodiment of the presentinvention comprises a bifurcated stent 150 formed from a single,ribbon-like piece of material 152. The material used may be a solidstrip of suitably deformable metal or plastic or other biocompatiblesubstance, or it may be a mesh, such as of woven metal threads. To forma single coil 154, a strip of material having a desired width is woundaround an axis 151. To form bifurcated stent 150, when the desiredlength of single coil 154 is attained, the remaining, uncoiled length ofthe strip is split lengthwise into two minor strips 155, 156, which areeach coiled into a smaller but similar coil 158, 160.

It will be understood that a bifurcated stent may be constructed bycombinations of the coil patterns disclosed herein other than thecombinations shown in FIGS. 5 and 9. For example, the coil of FIG. 1,which is shown as a closed loop, could be opened and combined with twominor coils such as coils 83, 85. Such a combination would form abifurcated stent from a single piece of wire with the wire endsterminating at the ends of the minor coils. These combinations do notdepart from the spirit of the invention.

The advantages of a stent coiled according to the above description, andin particular a bifurcated stent, are discussed below. Primarily, thepresent coil is an improvement on the art because the relative stiffnessof the regions of overlap and the turns are distributed longitudinallyevenly about the axis of the coils. This is advantageous, as it ispreferable that a stent not have a bias toward bending in one directionover another.

When a stent constructed according to the present invention is expandedinto its supporting state, the outside diameter of the coil increased bydecreasing the width of the regions of overlap. If the stent is expandedtoo much, the regions of overlap will disappear, as the intermeshedcusps will no longer overlap longitudinally. In the prior art thisresulted in a longitudinal gap in the stent, across which the bloodvessel was not supported. According to the present invention, even ifthe stent is expanded to such an extent that a gap is formed, the gap ishelical, winding around the length of the stent. It is believed that ahelical gap is preferable from a medical standpoint.

Because the stent of the present invention can be constructed from acontinuous loop of wire, it eliminates the wire ends that are commonlypresent on the stents of the prior art. Such wire ends must be bent intoloops, or otherwise treated, so as to decrease the likelihood ofpuncturing the vessel wall.

Double Spiral Stents

Referring now to FIG. 16, a stent 100 having a double-spiralconfiguration, as opposed to a double-helix, is shown. As discussedabove with respect to stent 10, stent 100 is preferably formed from asingle continuous loop 102, such as that shown in FIG. 19 and discussedbelow. Loop 102 is twisted about an axis 104, such that a cylindricalcoil having ends 106 and 108 and side sections 110 and 112 is formed.Each side section 110, 112 forms a spiral, with the two spirals beingdiametrically opposed except at ends 106, 108. Like stents 10 and 80,stent 100 provides uniform support around the circumference of a vessel.That is, it has no longitudinal gap. Unlike stents 10 and 80, however,stent 100 shortens or unwinds as it expands. This is a result of thespiral construction of, stent 100.

Again using spiral construction, a bifurcated stent 120, as shown inFIG. 17, may be formed. Bifurcated stent 120 includes a double-spiralmajor coil 122 and two double-spiral minor coils 124, 126. Stent 120 ispreferably formed from a single continuous loop 128, and major coil 122and minor coils 124, 126 are formed in the same manner as stent 100.

According to an alternate embodiment, shown in FIG. 18, loop 128 may becut at one end of major coil 122, and the resulting ends 130, 132 may beindividually coiled into single spirals. In this embodiment it ispreferred that the exposed ends be looped back and connected to theirrespective coils in order to reduce the possibility of their puncturingthe vessel in which they are installed, as at 134.

Methods of Making

According to the present invention, stents such as those described aboveare formed in a two-step process. The first step entails constructing acontinuous loop, or blank, in the desired shape, while the second stepentails forming the blank into a cylindrical coil. Methods for carryingout each step are described in detail below.

A. Forming a Continuous Loop or Blank

Continuous flat loops, or blanks, such as those shown in FIGS. 19 and20, are preferably used to form the stents disclosed above. According toa preferred embodiment, each blank is formed by a photo-etching process.In this process, a drawing is made of the desired blank shape. Thedrawing is then reduced to actual size. The reduced drawing is used toproduce a mask having the contours of the finished product. This mask isthen affixed to the substance to be formed, preferably by means of anadhesive. The masked substance is exposed to an etching chemical, whichcauses those portions of the substance that are not covered by the maskto be dissolved. The etching chemical is preferably an acid-basesubstance, such as are widely available. The selection and strength ofthe etching chemical will be determined by the composition of thesubstance to be etched.

Alternatively, the blank may be formed by laser-etching a sheet of thesubstance out of which the stent is to be constructed. Laser-etching isa conventional cutting technique, the details of which are well-known inthe art.

The substance that is etched can be any of various metals or metalalloys having the desired properties. For example, in some instances itmay be preferable to use stainless steel in the manufacture of thestent, because of its strength and corrosion resistance. In other cases,a memory metal, such as Nitinol, may be used.

If a memory metal is used to form the stent, its shape-retainingproperties can be used to advantage in two ways. First, if a stent inthe austenitic condition is formed into the desired stent shape andallowed to cool into martensite, it can be compressed or collapsed fordeployment. Collapsing of the stent deforms the martensite, whichretains the collapsed shape. The collapsed stent can be deployed andthen expanded by a brief application of heat, thereby obviating the needfor a balloon. This would be particularly advantageous in instanceswhere it is desirable to deploy and expand a bifurcated stent, as itavoids the need to position and then remove a Y-shaped balloon.

The second useful application of memory metals in vascular support iswhere it is desired to expand a vessel whose diameter has decreased toan unsafe degree. In these instances an austenitic stent is preformedsuch that it has a diameter slightly greater than that of the collapsedvessel into which it is to be deployed. The material is selected so thatit is "super-elastic" in the temperature range of the human body.Because the material is therefore springy and can be subjected tosignificant stress without deformation, the stent can be collapsed anddeployed without the use of heat. Because the material does not deformduring collapse, it must be contained in a catheter that is capable ofresisting the expansive spring force of the stent. Once deployed becausethe diameter of the stent is larger than the vessel diameter, the stentexerts a dilative radial force on the vessel wall. This causes thevessel to expand until the stent is fully expanded. This would eliminatethe present need to use balloons to expand a collapsed vessels and/orcollapsed stents.

Once etching is complete, the mask is removed, leaving only the cleanlycut metal blank 150, as shown in FIGS. 19 and 20. Each blank includes apair of ends 152 and a pair of legs 154 extending between ends 152.Because the starting material for the mask typically comprises a flatsheet, mask 150 will have a rectangular cross-section, as shown in FIG.19A. The thickness of the sheet of starting material will determine thethickness of blank 150.

It will be noted that the legs 154 of blank 150 shown FIG. 19 areessentially straight, except where they curve together at ends 152. Incontrast, each leg 154 of blank 150 of FIG.20 is cut into an inclined,serpentine shape. Each serpentine leg forms cusps 155, which aredesigned to intermesh when the blank is formed into a cylindrical coil.

Still another type of blank that can be etched and formed according tothe present invention is the rib-cage blank 160 shown in FIG. 21. Thisblank includes a longitudinal spine 162 with a plurality of looped ribs164 extending therefrom. Ribs 164 are preferably substantially U-shaped,as shown, in order to minimize turbulence in the fluid passing throughthe stented vessel, to maximize conformability of the stent to theinterior of the vessel, and to minimize the likelihood of a puncture.

Other methods for forming a continuous blank may alternatively beemployed. For example, two ends of a piece of wire may, be soldered orwelded and then machined to produce a smooth coupling. However thesemethods for forming are less desired because they require additionalmanufacturing steps and do not produce an integral piece as doesphotoetching.

B. Coiling the Blank into a Stent

Once blank 150 has been formed, it is shaped into its ultimatecylindrical form by one of the following methods.

Preferably, a plurality of secondary rollers 201-204 may be used asshown in FIG. 22 to wrap blank 150 around a drive roller or mandril 210.Secondary rollers 201-204 are positioned so that they do not contacteach other and are spaced slightly apart from and parallel to driveroller 210. The space between each secondary roller 201-204 and driveroller 210 is preferably equal to or slightly less than the thickness ofblank 150.

As shown in FIG. 22, blank 150 is fed at a steady rate into the gap 211between the first secondary roller 201 and drive roller 210. Therotating surfaces of these two rollers propel blank 150 toward thesucceeding gap 212 between secondary roller 202 and drive roller 210.The rotating surfaces of these two rollers in turn propel blank 150toward succeeding gap 213. In this manner, blank 150 is propelledcompletely around drive roller 210. The passage of blank 150 around onecircumference of roller 210 results in blank 150 assuming a cylindricalshape.

If blank 150 is fed at an oblique angle with respect to the axis ofdrive roller 210, as shown in FIG. 23, the opposite edges or sides ofblank 150 and the gap defined therebetween will form a longitudinalspiral around the drive roller. If a blank having straight sides isused, such as is shown in FIG. 19, a double-spiral stent like that shownin FIG. 16 will be formed. If, however, a serpentine blank is used, suchas is shown in FIG. 20, a double helix stent like that shown in FIG. 1will be formed. The positioning of cusps 155 of each leg 154 withrespect to cusps 155 of the other leg 154 will determine the pattern ofcircumferential support provided by the stent to the vessel. It has beenfound that an slanted serpentine blank like that of FIG. 20 will producethe desired double helix stent in which the longitudinal gap or regionof overlap between the cusps defines a spiral around the stent.

As above, rib-cage blank 160 may also be fed into the rolling mechanismto be formed into a cylindrical shape. Referring now to FIG. 25, ifblank 160 is fed into the rollers obliquely, spine 162 will describe aspiral and ribs 164 will intermesh to more completely define the desiredcylindrical shape. Alternatively, blank 160 may be formed into thedesired shape by other mechanical means.

It will be recognized by one skilled in the art that the foregoingmethod is superior to prior methods of forming a complex stent such asthe double-helix stent in that the present method allows rapid,consistent, and symmetrical assembly of the desired shape and is notlabor intensive. If, however, the necessary equipment is unavailable,the stents disclosed above may be formed "by hand," such as by manuallywrapping or pulling the blank into shape around a mandril.

While the foregoing stents described may be used alone, particularly ifthey are coated with a biocompatibilized gel as described above, it mayoptionally be desired to provide a casing for the deployed stent. Such acasing, or graft as they are called, may comprise a tube-shaped memberhaving an inside diameter only slightly larger than the circumference ofthe deployed stent. The casing may be made of latex, silicone latex,polytetraflouroethylene, polyethylene, dacron polyesters, polyurethaneor other suitable biocompatible material. The graft material must beflexible and durable, so that it can withstand the effects ofinstallation and usage. Depending on the material chosen, it may bepreferable to form the graft in one of several ways. For example, thegraft may be extruded, woven or formed by dipping a substrate in thedesired material, removing the material from the substrate, and trimmingthe end of the material, so as to form a cylindrical tube having anopening at each end.

The graft is deployed simultaneously with the deployment of the stent.Prior to deployment, the graft is collapsed, with the collapsed stentinside it. As described, the stent and craft may then be inserted into acatheter, deployed, and expanded by pressurization of a balloon. A graftdeployed and supported in this manner may be used to seal an aneurysm orsimilar defect in a vessel. The tissue of the vessel adjacent to thegraft will grow onto the graft, so that the graft becomes an integral,reinforcing that part of the vessel wall and helping to reduce the riskof future ruptures at that location.

While a preferred embodiment of the invention has been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit of the invention. It will further beunderstood that stents according to the present invention may be used inother body passageways, such as the urinary, biliary, or esophagealtract, with construction and deployment of the stents being essentiallyas described above.

What is claimed is:
 1. A method for making a stent having a first,radially compressed position and a second, radially expanded position,comprising the steps of:forming a continuous closed-loop blank, saidcontinuous closed-loop blank comprising two legs, each of said legshaving first and second ends, said first leg ends being joined at afirst stent end and said second leg ends being joined a second stentend, said legs being serpentine between said ends, and wrapping theblank helically around a mandrel so as to form a cylindrical coil inwhich the blank does not cross itself, the coil being configured suchthat when the stent is in its radially compressed position the arcuateribs formed by the serpentine legs overlap each other circumferentially.2. The method according to claim 1, further including the step offorming the blank by photoetching.
 3. The method according to claim 1,further including the step of forming the blank by laser etching.
 4. Themethod according to claim 1, further including the step of forming theblank from a memory metal.
 5. The method according to claim 1, whereinsaid wrapping step includes passing the blank between the mandrel and aplurality of secondary rollers.
 6. A method for making a stent,comprising the steps of:forming a blank, the blank including alongitudinal spine and a plurality of looped U-shaped ribs extendingtransversely from the spine, and wrapping the blank helically around amandrel so as to form a cylindrical coil that does not cross itself andin which the transversely extending U-shaped ribs overlapcircumferentially.
 7. The method according to claim 6, further includingthe step of forming the blank by photoetching.
 8. The method accordingto claim 6, further including the step of forming the blank by laseretching.
 9. The method according to claim 6, further including the stepof forming the blank from a memory metal.
 10. The method according toclaim 6 wherein said wrapping step includes passing the blank betweenthe mandrel and a plurality of secondary rollers.